Add .editorconfig for easier communication of code conventions.

Many editors support .editorconfig. From looking at some of the code in libgnomekbd (i've really only been playing around with the C code), it appears to be using the configuration directives I've documented here. The conventions documented here aren't my personally preferred conventions, but I find it it useful to let my editor know what the project uses, so that i can contribute without thinking too much about whitespace, etc.

If there are other conventions, this gives people a place to add them.

Or, if the project wants to update the conventions, it provides a place to document those changes.